1. Field of the Invention
The present invention relates to an X-ray image pickup device and, more particularly, to an X-ray image pickup device which has a large area and a high S/N ratio, and is applicable to a medical X-ray diagnostic apparatus.
2. Related Background Art
Nowadays, in an X-ray image pickup device is used medical diagnosis, which images the body by radiating X-rays onto a human body, capturing X-rays transmitted through the human body onto a phosphor that converts the X-rays into visible light, and exposing a film with the fluorescence (to be referred to as a film method hereinafter).
FIG. 1 shows the schematic arrangement of an X-ray diagnostic apparatus as a typical X-ray image pickup apparatus. Referring to FIG. 1, an X-ray source 901 radiates X-rays onto a patient, and the X-ray intensity and the radiation time are adjusted in correspondence with the purpose of diagnosis. In a human body (patient) 902, the radiated X-rays are scattered in every direction. For the purpose of increasing the resolution of an X-ray image, only X-rays in a specific direction are guided toward a phosphor (scintillator) 904 using a grid plate 903.
The scintillator 904 can generate fluorescence in a visible light region by recombination energy when the base material of the phosphor is excited (absorbed) and recombined by high-energy X-rays. The fluorescence is produced by the base material itself such as CaWO.sub.4, CdWO.sub.4, or the like, or by a luminescent center substance such as CsI:Tl or ZnS:Ag activated in the base material. Recently, phosphors using rare earth elements such as Tb, Eu, Pr, and the like as luminescent centers are used.
The grid plate is constituted by alternately arranging a substance (e.g., lead) that absorbs X-rays and a substance (e.g., aluminum) that transmits X-rays, and has the sectional arrangement shown in FIG. 2.
The fluorescence from the scintillator 904 exposes a film 905, and an X-ray image of the patient is obtained via a development process.
In another X-ray image pickup apparatus, a one- or two-dimensional CCD solid-state image pickup element is used in place of the film 905, so that the fluorescence from the scintillator 904 is imaged via a reduction optical system and the image is photoelectrically converted.
Not only in Japan in which the population of elderly people is increasing rapidly but also worldwide, improvement of diagnostic efficiency in hospitals and development of medical equipment with higher precision are strongly demanded. Under such circumstances, the conventional film type X-ray image pickup device has problems as described below.
Before a doctor acquires an X-ray image of a patient, a film development process must be performed, resulting in expenditure of much labor and time.
Sometimes, when a patient moves during X-ray image pickup or when the exposure amount is improper, image pickup must be inevitably redone. These factors inhibit improving the diagnostic efficiency in hospitals.
A clear X-ray image cannot often be obtained depending on the image pickup angle of the affected portion to be photographed. For this reason, in order to obtain an X-ray image required for diagnosis, some images must be taken while changing the image pickup angle. Such operation is not preferred especially when the patient is a baby or a pregnant woman.
Furthermore, X-ray image films must be preserved after image pickup for a certain period of time in hospitals, and the number of such films becomes very large in hospitals, resulting in poor efficiency in terms of management since the films must be pulled and replaced every time a patient comes to a hospital. When a patient needs to change the hospital he or she normally visits to seek medical attention for some reason; for example, when a patient in a remote place must undergo diagnosis as highly advanced as that he or she can receive only in a midtown university hospital or must move abroad, then X-ray films (after exposure and development) must be delivered to the next hospital by some method. Otherwise, the patient must be subjected to an image pickup operation again in the new hospital.
These problems are serious obstacles against establishing a new system of medical practice in future. It is the first object of the present invention to solve the above-mentioned problems.
In recent years, in medical industries, demand for "digitization of X-ray image information" is increasing. If the digitization is attained, X-ray image information can be managed using recording media such as magneto-optical disks, and a doctor can acquire X-ray image information at an optimal angle in real time. When a communication system such as a facsimile system, and the like are utilized, X-ray image information can be sent to hospitals everywhere in the world within a short period of time. Furthermore, when the obtained digital X-ray image information is subjected to image processing using a computer, diagnosis with higher precision than in the conventional method can be realized, and all the problems that the conventional film method has encountered can be solved.
Recently, an X-ray image pickup apparatus that uses a CCD solid-state image pickup element in place of a film has also been proposed.
However, existing CCD solid-state image pickup elements cannot be manufactured in a size that matches human bodies. When a CCD solid-state image pickup element is used, fluorescence, i.e., an X-ray image, from the scintillator must be imaged on the CCD light-receiving surface via a reduction optical system. It is difficult to attain an effective size reduction of the reduction optical system including the lenses which results in a large X-ray image pickup apparatus.
Since an X-ray image is formed via a lens, light components that do not enter the lens are wasted. Therefore, light components entering a lens are reduced to 1/100 to 1/1,000 although such ratio depends on the reduction ratio, and it is generally accepted that the S/N (signal to noise) ratio is reduced to 1/10 to 1/100 upon passing through the lens. This fact is expected to be disadvantageous upon applying the CCD solid-state image pickup element to medical equipments that require high gradation characteristics.